The field of the present invention is the sifting of airborne materials.
In the handling of fine materials such as flour, pneumatic conveying is often employed. Additionally, sifting is customarily performed on such materials during processing to remove foreign debris and to insure proper size distribution of the material itself. Centrifugal and round horizontal sifters are commonly used in such operations. Typically centrifugal type separators are employed with the lower capacity applications while horizontal separators are used for higher flow rates of air entrained material.
With horizontal separators, sifting occurs through screens mounted in a closed housing. The screens are vibrated to improve efficiency. Preferred screening conditions are understood to be achieved when the conveying air is reduced in velocity. With decreased turbulence, the material is allowed to settle on a screen and pass therethrough. To accomplish this, the cross-sectional area of flow must be increased substantially within the sifter. Because of the pneumatic conveyance and the closed nature of such housings, accommodation of overpressure must also be considered.
A relatively simple system has been practiced which employs a vibrated horizontal sifter screen with no air bypass. Airborne material is presented to the sifting chamber. The chamber is substantially larger in cross section than the conveying passage leading thereto. Consequently, velocities are reduced substantially. However, flow of both the carrier air and material must pass through the vibrating screen. Such devices are relatively inexpensive, are easy to service with easy disassembly and reassembly, are relatively small and require a minimum of dynamic forces for vibration. However, such devices are subject to overpressure which can cause screen damage and potentially less efficient screening.
A means for avoiding such overpressures includes devotion of a portion of the housing to air separation from the entrained material and bypass of that separated air. To accomplish such air bypass, the flow must be slowed very substantially to allow the entrained material to fall from the air flow. Once gravity separation has occurred, the air flowing through the housing is channeled around the sifting screens to where the sifted material may be reentrained and conveyed from the equipment. Such systems prevent the possibility of overpressure through use of the air bypass. However, increased size, complexity of disassembly and reassembly, cost and high dynamic forces compromise the bypass advantage.